1. Field of the Invention
This invention relates to an image forming apparatus which jets ink to adhere on a recording medium and to form an image.
2. Description of the Related Art
Heretofore, various non-impact recording type image forming apparatuses including electrophotographic systems have been developed. Among these non-impact recording systems, on-demand type ink jet recording systems, which are advantageous in price reduction, reduction of running cost, minimization of the apparatus, and low noise level, have drawn attention, and various types of ink jet recording apparatuses have been developed and commercialized.
A typical on-demand type ink jet recording apparatus is provided with a passage for supplying ink, a nozzle or an aperture for jetting ink, and a mechanism for generating a pressure to jet ink. In particular, the thermal type which uses a heat generating element and the piezoelectric type which uses a piezoelectric element as a mechanism for generating the pressure have been known as the typical on-demand types. In the thermal type, ink droplets are jetted from the nozzle head with a vapor pressure generated by heat generated from a heater provided in an ink passage and adheres on a recording medium, and an image is formed. For example, Japanese Published Unexamined Patent Application No. Sho 54-51837 and Japanese Published Unexamined Patent Application No. Sho 54-59139 disclose such an on-demand thermal type ink jet recording apparatus. In the piezoelectric type, the volume of a passage is changed by displacement of a piezoelectric element provided in the passage, ink is jetted from the nozzle head by the pressure and adheres on a recording medium, and an image is formed. For example, Japanese Published Unexamined Patent Application No. Sho 53-12138 and Japanese Published Examined Patent Application No. Sho 53-45698 disclose such an on-demand piezoelectric type ink jet recording apparatus.
In the conventional ink jet system in which ink droplets are jetted from a nozzle or an aperture, ink having low viscosity in a range from 1 mPa.multidot.s to 3 mPa.multidot.s is used, and ink is jetted from a nozzle or an aperture having a small diameter in a range from 30 .mu.m to 50 .mu.m. Because the viscosity of ink is low, the reduction of the ink quantity in the nozzle or aperture due to ink jetting is re-filled at high speed from an ink storage unit. Therefore, because the meniscus (interface between ink and air) in the nozzle head or aperture resumes rapidly, ink jetting is stable even though high speed printing at a repeating jetting frequency exceeding 10 kHz is performed.
However, in the case that low viscosity ink is printed on a normal paper, fast ink penetration causes feathering along fibers of the paper, the feathered image feels rough and results in reduced image quality. When adjacent dots are concatenated due to feathering, the resolution is reduced on that portion, and the image quality is deteriorated severely. Suppression of penetration speed by controlling the surface tension of ink causes inter color bleeding with ink of different color which is jetted on the adjacent spot, also resulting in deteriorated image quality severely.
To solve the above-mentioned problem caused from low viscosity ink, the use of high viscosity ink helps the penetration speed of the ink into a recording medium to be slowed and prevents feathering, and also helps the bleeding speed between different colors to be slowed and prevents bleeding, and the use of high viscosity ink is very effective for solving the above-mentioned problem.
With the extensive diffusion of recording apparatuses using ink jet system, application is widening rapidly, and therefore the wide variety of recording media have been used. It is desired that the ink jet recording system is applied not only to paper but also to various non-water absorptive media such as metals, plastics, and glasses, and it is desired that the ink jet recording apparatus which is capable of using high viscosity and tacky ink is realized.
However, it is difficult for the conventional ink jet system having a nozzle or an aperture to make use of high viscosity ink because the passage resistance is high and the ink supply speed is very low. With an increase of the passage resistance, the energy required to jet ink increases. For example, if an ink jet apparatus which normally uses commercially available water color ink (having a viscosity in a range from 1 mPa.multidot.s to 3 mPa.multidot.s) uses high viscosity ink, the ink supply becomes insufficient if the ink viscosity is higher than 10 mPa.multidot.s, and the resumption of meniscus cannot follow the ink consumption. Ink jetting in such state results in scattered ink jetting quantity and jetting direction and results in poor picture quality because ink is jetted from the unstable meniscus position. When ink having a viscosity exceeding 20 mPa.multidot.s is used, the ink supply delays from consumption severely, and irregular jetting is caused. Further, when ink having a viscosity exceeding 100 mPa.multidot.s is used, even if the supply can follow the consumption, the energy generated from the conventional pressure generation unit to jet the ink is insufficient, the ink is jetted unstably or no ink is jetted.
To solve such a problem, for example, Japanese Published Unexamined Patent Application No. Hei 9-169111 proposes a printer in which a nozzle having a large diameter in a range from 50 .mu.m to 70 .mu.m is used to reduce the passage resistance, and high speed printing is realized in spite of using high viscosity ink having a viscosity in a range from 10 mPa.multidot.s to 100 mPa.multidot.s. In this method, the ink supply delays from the ink consumption and repetition printing frequency decreases unless the larger nozzle diameter corresponding to the incremental ink viscosity is used. However, the larger nozzle diameter results in a relatively increased diameter of jetted ink droplets and results in reduced resolution and poor image quality.
To solve the above-mentioned problem, for example, Japanese Published Unexamined Patent Application No. Sho 62-90257 proposes a method in which solid ink is heated so that the viscosity is reduced to a low viscosity in a range from 5 mPa.multidot.s to 10 mPa.multidot.s, and the ink is jetted in the same manner as used in a normal ink jet system, the ink is cooled and solidified immediately when the ink adheres on a recording medium, and recording is performed. However, in this method, the head unit should be heated always, such heating results in increased energy consumption. Further, because ink droplets are solidified at the moment when the ink droplets adhere on a recording medium, the ink scarcely penetrates into the recording medium, and such insufficient penetration causes a fixing problem. In addition, ink droplets are solidified in the form of a semisphere, and such a semisphere results in the rough surface of the image. The light is scattered on the image surface, the glossiness of the image is reduced, and the color turbidity of OHP is caused, and therefore this method involves many problems.
Further, for example, Japanese Published Unexamined Patent Application No. Sho 56-4467 proposes a method in which ink is jetted from a nozzle or a slit aperture by attracting the ink electrostatically. This method is advantageous in that the ink supply is significantly independent of ink viscosity because the ink is jetted and supplied simultaneously and the ink droplet diameter is not very dependent on the nozzle diameter or the slit width. However, the high viscosity causes long stringing when the ink is attracted, and it is difficult to form fine dots. Further, satellite drops are generated from the stringing portion, and the image quality becomes very poor. If ink having higher viscosity is used, the cohesion of the ink is larger than the electrostatic attractive force, the printing becomes impossible.
Some ink jet systems having no nozzle have been proposed though they are not commercialized. For example, Japanese Published Unexamined Patent Application No. Sho 51-132036 proposes a method in which many small electric heaters are provided just under an ink level of an ink layer having the depth of 300 .mu.m, a current is supplied selectively to heaters located at specified positions to form bubbles, the bubbles burst at the ink surface to generate fine droplets of the ink, and the fine droplets are used for recording. In this method, it is considered that low viscosity ink is used, and actually, it was confirmed that an ink droplet jetting phenomenon was observed when low viscosity ink having a viscosity in a range from 1 mPa.multidot.s to 3 mPa.multidot.s was used.
However, when ink having higher viscosity is used, though the ink is jetted for the ink having a viscosity up to 10 mPa.multidot.s, ink droplets split when bubbles burst, fine droplets other than droplets served for recording are scattered to the surrounding areas, so-called misting is caused, and the image quality is deteriorated severely. It is considered that high ink viscosity results in high internal resistance in the ink and prevents bubbles from growing. Based on the ink jetting principle of this method, it is required for the ink thickness to be thick to some degree, but because the internal resistance force depends on the ink thickness, high viscosity ink cannot be used because of increased internal resistance force. In the method, ink is supplied based on the fluidity of the ink, therefore, if the fluidity decreases with an increase of ink viscosity, the ink is not supplied swiftly enough for the ink consumption.
Yet another method, for example, Japanese Published Unexamined Patent Application No. Sho 62-184860 proposes a method in which commercial water color ink having a thickness in a range from 20 .mu.m to 80 .mu.m spread on a belt support is conveyed, the ink or support is heated by a laser beam to generate bubbles, and ink droplets are jetted by bursting the bubbles in the same manner as described in the Japanese Published Unexamined Patent Application No. Sho 51-132036. In comparison with the above-mentioned Japanese Published Unexamined Patent Application No. Sho 51-132036, this method is advantageous in that fluctuation of bubble generation position is suppressed and bubble generation efficiency is improved. Further, because ink spread on the support is conveyed continuously in this method, the ink is supplied swiftly enough for the consumption even though high viscosity ink is used.
In this method, commercially available ink is used as the ink, the viscosity of commercially available ink is as low as 1 mPa.multidot.s to 3 mPa.multidot.s. For example, if ink having a viscosity up to 10 mPa.multidot.s is used, the vibration of the ink is transmitted to the surroundings when the ink is jetted. As the result, when the ink corresponding to the adjacent pixel is jetted, the vibration of the ink level due to transmission from the adjacent pixel which is previously jetted affects to cause unstable jetting direction and jetting quantity. In addition, when low viscosity ink is jetted, ink flows onto the blanked portion where the ink is jetted to decrease from the surroundings after the ink is jetted, and the thickness of the ink is uneven when the ink corresponding to the adjacent pixel is jetted and the jetting quantity becomes unstable. Further, as in the case of the above-mentioned Japanese Published Unexamined Patent Application No. Sho 51-132036, ink droplets split into pieces when bubbles burst because of low viscosity, and misting is caused. In addition, because the ink to be jetted receives a tensile force from the peripheral ink when the ink is separated as an ink droplet, a large amount of energy is required to jet the ink. When higher viscosity ink is used, the ink is not jetted though small bubbles are generated.
Yet another method, for example, Japanese Published Unexamined Patent Application No. Sho 61-118273 proposes a method in which ink held on a holding member having many pits for holding ink is conveyed, the ink is heated using a laser beam, and the ink is jetted by the pressure of bubbles generated in the ink. The same method is described in FIG. 5(A) and FIG. 5(B) of the above-mentioned Japanese Published Unexamined Patent Application No. Sho 62-184860. FIG. 25(A) to FIG. 25(E) are diagrams for describing one example of a conventional ink jet recording system having no nozzle. As shown in FIG. 25(A) to FIG. 25(E), the ink held in a pit is in contact with the side wall of the pit, and the contact causes resistance. When a bubble is generated by heating the ink from the bottom of the pit with starting from the state shown in FIG. 25(A), the ink is extruded from the pit as the bubble grows, and then, the ink receives a resistance from the side wall of the pit. As the result, a large amount of energy is required for jetting. Particularly, the higher the ink viscosity is, the larger is the resistance between the ink and the side wall of the pit, and the more energy is required.
Just before the bubble bursts after the bubble grows, the ink is in contact with the pit at the precise image as shown in FIG. 25(C). If there is slight deviation when the bubble bursts, the ink partially remains adherent to a portion of the edge of the pit as shown in FIG. 25(D) and FIG. 25(E), and the jetting direction of the ink droplet is changed due to adherent force of the ink and the image quality becomes poor. Such adhesion of the ink just before jetting depends on the shape of the pit and the surface condition of the holding member on which the pit is formed, and it is impossible to control the jetting direction unless the shape and the surface condition are perfectly uniform.
Though bubbles burst and ink is jetted using ink having the viscosity up to 10 mPa.multidot.s, misting is severe.
To solve the above-mentioned feathering and bleeding problem, various methods in which the recording medium and ink characteristics are devised to solve the problem have been proposed. As a method in which the recording medium is devised to solve the problem, for example, Japanese Published Unexamined Patent Application No. Hei 9-207429 proposes a method in which a polymer porous layer is formed on a synthetic paper base, ink is absorbed and held in the polymer porous layer in a moment, in order to solve the above-mentioned problem. As a method in which the ink characteristics is devised to solve the problem, for example, Japanese Published Unexamined Patent Application No. Hei 8-281932 proposes a method in which plural kinds of ink having different properties are used, chemical reaction is caused by contact of the plural kinds of ink having different properties, they are fixed in a moment, in order to solve the above-mentioned problem.
However, the method in which the recording medium is devised results in high cost of the recording medium itself, the recording medium is limited to the special grade dedicated paper, and the method is used only in limited application as the recording system. The method in which the ink characteristics are devised results in high cost of the ink itself, a process which requires plural kinds of ink having different properties is complex, and leads to a large-scale and high cost apparatus.
As described hereinbefore, the conventional ink jet recording technology cannot bring about high speed stable jetting of a small amount of high viscosity ink, therefore, cannot solve the problem of feathering of ink and bleeding between different colors, and cannot realize wide application of ink jetting onto the variety of recording media.
The present invention has been accomplished in view of the above-mentioned problem, the object of the present invention is to provide an image forming method which is capable of stable jetting of a small amount of high viscosity ink at high speed without incremental energy consumption and to realize an image forming apparatus which is capable of forming high quality highly precise images at high speed without feathering on various recording media.